essay 23 - composite materials - classifications, types, composition, properties, indications

what is composite?

• a physical mixture of different materials chosen to produce intermediate or optimised properties. In dental composites: the matrix phase (resin) is continuous and fluid during application. The filler phase (silica particles) is dispersed, reinforcing the material

describe some advantages of composite restorations

• minimally invasive and high aesthetic

• time and cost effective compared to porcelain crowns

• less ideal than ceramics for severely weakened teeth under heavy occlusion

classifications of composites

1. Filler particle size

• heterogenous e.g hetero-microfill

• homogenous e.g nanofill, microfill, minifill

• hybrid e.g midi-microfill, mini-nanofill

2. polymerisation method

• QTH: quartz tungsten halogen

• LED: light emitting diode - most common

• Plasma arc curing

• Laser curing - nor common as its very expensive

types of composites

1. Conventional composites

2. Microfill composites

3. Hybrid composites

4. Nanofill composites

5. flowable composites

6. packable composites

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describe conventional composites

• 75-80% filler by weight

• large, hard particles → rough surface after wear

• more prone to extrinsic staining

• often replaced by hybrids

describe microfill composites

• -35-60% filler

• use colloidal silica → smooth, enamel like finish

• inferior mechanical properties but excellent aesthetics

• ideal class V cervical lesions (flexibility during tooth flexure )

describe hybrid composites

• 75-85% filler

• mix of microfill + small particles

• good strength and polishability

• most commonly used aesthetic restoratives

describe nanofill composites

• extremely small fillers (nanometers)

• high polishability and excellent physical properties

• increasing popularity due to aesthetic and mechanical performance

describe flowable composites

• lower filler content → less strength and wear resistance

• used in thin layers, sealant, liners, or small class I restorations

• high polymerisation shrinkage → caution in bulk use

describe packable composites

• high viscosity → amalgam line handling

• developed for proximal contacts

• “line handling” concept refers to the controlled, stepwise placement, packing, and adaptation of the composite in increments to avoid voids, ensure marginal seal, and reproduce anatomy

composition of composites

1. Matrix phase

• originally based on methyl methacrylate, later replaced by BIS-GMA

2. Filler phase

• Mainly modified silicate glass (silica)

• enhances:

— strength

— wear resistance

— optical properties (enamel like translucency)

• silane coupling agents chemically bond filler to resin:

— one end binds to silica; the other co-polymerises with the matrix

• fillers may include ions like

— barium, zinc, zirconium (radiopacity)

— lithium, aluminium (crushability)

• filler size affects fluidity and polishability

— smaller particles → more surface area → less flow and better finish

properties of composites

1. Thermal expansion (LCTE)

• improved composites: -3x tooth structure

2. water absorption

• all composites absorb water to some extent. More filler = less absorption

3. wear resistance

• affected by particle size, shape and content

• generally good; approching amalgam performance in may cases

4. Surface texture

• influenced by filler composition

• microfills = smoothest, but hybrids also polish well

5. radiopacity

• achieved with barium, strontium or zirconium fillers

• allows easy radiographic defection of caries

6. modulus of elasticity

• microfills = more flexible → better in class V with flexure

• Hybrids - more rigid → suited for stress bearing areas

7. solubility

• clinically negligible in modern composite

8. Polymerisation shrinkage

• always present during curing

• can cause marginal gaps, especially at root surfaces

• Minimised using:

— incremental curing

— stress breaking liners

— proper bonding protocols

indications for use of composites

• teeth which can be isolated

• teeth experiences normal occlusal loading and occlusal contacts

• patients requiring high aesthetic

• anterior teeth